1. Field of the Invention
The present invention relates to a PWM inverter control system and method enabling a PWM inverter or PWM inverters connected in parallel to supply stable power.
2. Description of Related Art
Referring to FIGS. 1 and 2, a conventional method for controlling a PWM inverter or PWM inverters will be described.
First, voltage control of a single operation of a PWM inverter 1 will be described. In FIG. 1, a PWM inverter 1 is controlled by a control circuit 33, and the output of the PWM inverter 1 is supplied to a sine filter (LC low-pass filter) 23. The sine filter 23 comprises an LC filter composed of a reactor 25 and a capacitor 26, and a damping circuit 27 which is a serial circuit of a resistor 28 and a capacitor 29. The damping circuit 27 is connected in parallel with the capacitor 26 in order to limit oscillation waveforms accompanying the resonance of the reactor 25 and the capacitor 26. The control circuit 33 comprises a mean value circuit 2, an automatic voltage regulator (AVR) 3, an instantaneous voltage command value generator 7, and a PWM signal generator 9.
The PWM inverter 1, the sine filter 23, and the control circuit 33, together with a rectifier 51 can construct a conventional uninterruptible power supply 50 as shown in FIG. 3.
A method for controlling the PWH inverter is as follows:
First, a voltage detector 12 is connected to the output of the sine filter 23 to detect the instantaneous output voltage V of the sine filter 23. The output voltage V is inputted to the mean value circuit 2. The mean value circuit 2 produces the mean value V.sub.A of the instantaneous output voltage V. The mean value V.sub.A is subtracted from a predetermined voltage reference value V.sub.A * by a summing point 81, and the difference .DELTA.V.sub.A is supplied to the automatic voltage regulator 3. The automatic voltage regulator 3 corrects the voltage reference value V.sub.A * so that the difference .DELTA.V.sub.A becomes zero, and supplies the resultant corrected voltage reference value V.sub.A ** to the instantaneous voltage command value generator 7. The instantaneous voltage command value generator 7, receiving the corrected voltage reference value V.sub.A ** and a predetermined frequency reference value .omega.*, generates a sinusoidal instantaneous voltage command value V* having an amplitude determined by the corrected voltage reference value V.sub.A ** and a frequency determined by the frequency reference value .omega.*.
Furthermore, the output voltage V is subtracted from the instantaneous voltage command value V* by a summing point 82, and the difference .DELTA.V is inputted to a gain adjuster 45. The output of the gain adjuster 45 is added to the voltage command value V* by a summing point 83. The summing point 83 outputs a corrected voltage command value Va*, and supplied it to the PWM signal generator 9. The PWM signal generator 9 outputs a pulse signal corresponding to the corrected voltage command value Va*, and controls the PWM inverter 1 by the pulse signal.
The operation of an inverter control system controlling a plurality of PWM inverters connected in parallel to supply power to a common load will be explained.
FIG. 2 shows this type of control system. In FIG. 2, the output of the inverter 1 is connected to the sine filter 23, and the PWM inverter 1 is controlled by a control circuit 34.
FIG. 4 shows a power supply system including a plurality of uninterruptible power supplies 50. Each uninterruptible power supply 50 is constructed as shown in FIG. 3 using the PWM inverter 1, the sine filter 20, the control circuit 30, and the rectifier 51. The outputs of individual uninterruptible power supplies are connected to a bus board 60, which will be described later, to carry out the parallel operation through the bus board 60, and the output of the bus board 60 is supplied to a load 70. Specifically, the output of each PWM inverter 1 is connected to the bus board 60 through the sine filter 23, and the output of the bus board 60 is connected to the load 70. Here, a method for controlling a single PWM inverter in the plurality of PWM inverters will be explained.
In FIG. 2, a current detector 11 is provided between the sine filter 23 and the bus board 60, and the voltage detector 12 is connected to the output of the sine filter 23. The current detector 11 detects the instantaneous output current I.sub.L flowing out of the sine filter 23, and the voltage detector 12 detects the output voltage V. The other PWM inverters are connected in parallel with the PWM inverter 1 in the bus board 60. The bus board 60 controls distribution of power to a load, and supplies the control circuit 34 of the PWM inverter 1 with an instantaneous output current command value I.sub.L * which is determined in accordance with the number of PWM inverters operated in parallel. In FIG. 2, reference numerals 42, 43 and 44 designate filters that remove noise contained in a detected value and command values, respectively.
The control circuit 34 includes, in addition to the control circuit 33 shown in FIG. 1, a reactive component synchronous rectifier 5, an active component synchronous rectifier 6, an automatic frequency regulator 4, and summing points 84 and 85. The control circuit 34 controls the PWM inverter 1 in accordance with the output current I.sub.L detected by the current detector 11, the output voltage V detected by the voltage detector 12, and the instantaneous output current command value I.sub.L * produced from the bus board 60.
More specifically, the control circuit 34 obtains a cross current I.sub.ou flowing between the PWM inverter 1 and the other PWM inverters by subtracting the output current I.sub.L from the output current command value I.sub.L * by the summing point 84. In other words, the cross current I.sub.ou is given by the following equation: EQU I.sub.ou =I.sub.L *-I.sub.L ( 1)
The cross current I.sub.ou is passed through the filter 44, led to the reactive component synchronous rectifier 5 and the active component synchronous rectifier 6, and is synchronously rectified. The reactive component synchronous rectifier 5 obtains the reactive component sin.phi., whereas the active component synchronous rectifier 6 obtains the active component cos.phi.. The active component cos.phi. is supplied to the automatic frequency regulator 4. The automatic frequency regulator 4 controls the gain by the proportional plus integral action. Its output is subtracted from a predetermined frequency reference value .omega.* by the summing point 85, thereby giving a frequency command value .omega.**. Furthermore, the difference between the mean value V.sub.A of the output voltage which is described referring to FIG. 1 and the predetermined voltage reference value V.sub.A * is further corrected by the reactive component sin.phi. by the summing point 81, and the corrected difference .DELTA.V.sub.A is inputted to the automatic voltage regulator 3. The automatic voltage regulator 3 controls its gain by the proportional plus integral action, and outputs a mean voltage command value V.sub.A **.
The instantaneous voltage command value generator 7, receiving the frequency command value .omega.** and the mean voltage command value V.sub.A **, outputs a sinusoidal instantaneous voltage command value V* having an amplitude determined by the mean voltage command value (that is, corrected voltage reference value) V.sub.A ** and a frequency determined by the frequency command value .omega.**.
Furthermore, the difference .DELTA.V between the voltage command value V* and the output voltage V is obtained by the summing point 82, and is inputted to the gain adjuster 45. The output of the gain adjuster 45 corrects the voltage command value V* by the summing point 83, and the corrected voltage command value Va* is inputted to the PWM signal generator 9. The PWM signal generator 9 provides the PWM inverter 1 with a pulse train controlling the PWM inverter 1.
In the conventional power supply, the sine filter 23 is provided to remove switching ripples involved in the PWM control regardless of a single or parallel operation of the PWM inverter(s). The sine filter 23 includes the damping circuit 27 composed of the resistor 28 and the capacitor 29, which is connected in parallel with the capacitor 26 as shown in FIGS. 1 and 2, in order to damp the oscillation of the reactor 25 and capacitor 26. Eliminating the resistor 28 and capacitor 29 to reduce the size of the sine filter will induce the resonance of the reactor 25 and capacitor 26 in the conventional system, resulting in the deterioration of the output voltage waveform of the PWM inverter 1 owing to the oscillation waveforms accompanying the resonance.
The resistor 28 and capacitor 29, however, present a problem in that they complicate the circuit configuration. In addition, since a part of a main current flows through the damping resistor 28, a large resistor is required as the resistor 28. This will cause the loss due to the damping resistor and the increase in size of the sine filter 23.
In addition, since the outputs of the PWM inverters are connected in parallel, the voltage sources of the PWM inverters are shortcircuited by output cables. Therefore, the instantaneous voltage detected by each PWM inverter is the mean voltage of individual PWM inverters. For this reason, imbalance of output sharing between the PWM inverters cannot be detected.
As a result, a voltage source supplying the current flowing through the capacitor 26 constituting the sine filter 23 cannot be identified. This will disturb the balanced current sharing among the PWM inverters in the parallel operation.